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Is it normal to see 76 volts above the ground on the positive leg of the heater of the first kt120 tube after relay is closed? The potential difference between the positive and the negative legs of the heaters (both KT120s included) is 13 volts. Nothing wrong there. But 13 volts potential difference is maintained with 76 volts on the positive leg and 63 volts on the negative leg (measurement reference is the ground) if I remember correctly.

That is done on purpose. Bruce always uses a reference voltage on the heater to avoid possible arcs and shorting inside the tubes especially on the SRPP stage. In the SRPP circuit it is easy to exceed the heater to cathode voltage ratings of the tube, causing an internal short. You should never ground the heater circuit on such design, doing so defeats the purpose of elevating the heater voltages

Thank you. The interesting thing about this problem is that it does not occur everytime the amplier is turned on. Should I suspect 0.2 uF cap between pin 1 and 2 of lm555? Cause, mine is a very tiny one. If it is short, is it possible to see this problem?

ozapaydin wrote:

Houston, I have a problem. The relay is closing after about 50 seconds but opens again after a few seconds. What might be the problem?

Hi, I used a dual pole double throw relay on the timer circuit. One pole switches on the b+ the other is connected on no on a lm358 breather circuit, the other side to cathode. So when the timer is still counting the b+ is not connected and the led is connected to the breather, once the timer reaches one minute the b+ is connected and the led bypass the breather so it stands on still.

For the pictures the hosting website must have removed it, gonna upload some in a sec

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